The present invention relates to a smoke detector, and more particularly to an ionization-type smoke detector.
An ionization-type smoke detector has been known in which an ionization chamber is provided to detect the presence of smoke, and when smoke enters the ionization chamber the change in the ion current therein is transformed into an alarm signal, issuing a fire alarm.
Although it has been known in a smoke detector of this kind to provide only one ionization chamber, this has such a drawback in that its sensitivity is affected by changes in characteristics in regards to change in environmental temperature, or gradual increases in smoke concentration.
To avoid such a drawback, an ionization-type smoke detector has already been proposed in which in addition to an outer ionization chamber into or out of which environmental air is free to enter or exit, there is an inner ionization chamber which has a hermetically sealed mechanism independent from the environment, the inner electrode within the inner ionization chamber mounting a radioactive source, the radiation of the radio active source able to penetrate into the outer ionization chamber through a through-hole formed axially of the intermediate electrode. Further, it has also been proposed that in order to prevent the radioactive source being contained in the detector from scattering outwards when the outer case made of synthetic resin is melted due to a fire, the radioactive source be encased within a metallic case so that it can fall to the floor together with the case.
Thus, it will be understood that hitherto known ionization-type smoke detectors have required a detector base board having a complicated construction, and since the main components of the detector such as the inner, intermediate and outer electrodes are mounted on the base board by screws, pins, etc. it has a complicated construction, requiring long assembly time, making for poor productivity.
Further, since the shield case and the case for preventing the radioactive source from scattering outside in case of the fire are independently made of metal or other suitable material the cost of the detector becomes more expensive.